The invention relates to a method for determining variations in the properties of a sample exposed to artificial weathering, in particular consisting of a polymer material irradiated in a spectral range corresponding to solar radiation.
Methods for rapid weathering with commercial weathering equipment are known from, for example, DE 28 16 548 A1, EP 0 320 209 A2, DE-AS 1 904 097, U.S. Pat. Nos. 4,760,748, 4,874,952, DE 25 02 239 B2 or DE 35 04 793 C2. The known equipment is used for simulation of the ambient conditions, with the irradiation intensity and temperature being set to the maximum of the respective operating conditions. This permits a time acceleration that can have factors from 10 to 20 depending on the material. From GB 21 74 800 A, a device is known using which the radiation reflected from a lamp is continuously measured in order to ascertain color changes. The sample itself is disposed on a metal block that is set to a required temperature.
To permit a rapid analysis of plastics, a method and a device are proposed according to DE 43 31 296 A1 in which the plastic to be determined is irradiated with laser light, in order to then identify ablation/evaporation products using a Fourier transform/infra-red spectrometer.
To determine the absorption and laser resistance of optical layers, samples are arranged on a sensor and irradiated in accordance with DE 41 09 469 A1.
In order to determine the aging state of plastic objects regardless of their age, it is proposed in accordance with DE 31 21 928 A1 to illuminate the plastic objects with a light source that emits light in the entire visible spectral range, and then to determine the remission at at least one point on the surface of the plastic objects.
To rule out to a large extent an inadmissible temperature increase during the determination of variations in the properties of samples due to photochemical processes, it is known fromxe2x80x94for examplexe2x80x94DE 34 43 694 A1 how to cool the sample with air currents and fans or similar. With the known method, it is then possible to establish color differences, by a comparison of before and after the irradiation.
DE 40 02 985 A1 describes measures for avoiding unwelcome heating up of a sample during irradiation in a spectral range corresponding to solar radiation and with an irradiation intensity which is more than 30 times greater than that of solar radiation, by passing an air current over the surface of the sample.
According to DE 42 10 585 A1, the absorbed radiation used for assessing the aging process of the sample is calculated from a measured spectral absorption capacity of a sample and from the spectral distribution of the solar radiation.
Regardless of the large number of known methods and devices for rapid weathering of objects, the achievable acceleration factors are limited, in particular when it must be avoided that the sample is heated to an extent that other temperature-dependent influences occur in addition to photochemically determined aging processes. Furthermore, the samples are on principle not inspected until the treatment is complete.
The problem underlying the present invention is to develop a method for determining of variations in the properties of a sample exposed to artificial weathering such that high acceleration factors are achieved, in order to minimize the inspection times for quality checks. It should at the same time be assured that variations in the properties resulting from unwelcome temperature increases in the sample from radiation impinging thereon are ruled out or the heating of the samples can be set in a defined way regardless of the photochemically effective radiation, for example by additional IR radiation.
The problem is solved in accordance with the invention by a method for determining variations in the properties of a sample exposed to artificial weathering, in particular a sample of a polymer material that is irradiated in a spectral range corresponding to solar radiation and with an irradiation intensity that is at least about five times greater than that of solar radiation in the corresponding spectral range, with the variations in the properties of the sample being measured during the action of the radiation.
In a fundamental divergence from the prior art, the variation of the property of the sample during the artificial weathering is tracked by preferably continuous measurement of the transmitted or reflected radiation, for example by means of a UV spectral radiometer or another instrument for measurement of the variations in the properties. Here the method in accordance with the invention is suitable in particular for transparent or semi-transparent samples, i.e. directly applicable for films, free lacquer films or transparent covering lacquers. In the case of opaque samples, the change in the optical properties can also be measured in reflection. Alternatively, and as a further development worthy of emphasis, thin and still sufficiently transparent samples could be prepared from opaque materials.
In particular, the invention provides that the sample is irradiated with radiation in a wavelength range between approx. 295 and 400 nm, preferably between 295 and 320 nm. The sample can here be irradiated with an irradiation intensity corresponding to approximately ten times the solar radiation in the corresponding spectral range, without any marked temperature increase in the sample itself occurring. Here both the spectral range and the irradiation intensity should be set such that the sample is irradiated with a radiation in the UV wavelength range and with an intensity such that the sample is heated less than 3xc2x0 C. during irradiation. Alternatively, the spectral range simulating solar radiation can be extended to 400 nm. As a result of the higher irradiation intensity, the heating up of a black body is then about 10xc2x0 C.
With the teachings in accordance with the invention, unexpectedly high acceleration factors are achievable that permit a very rapid yet precise quality control, since falsifying influences in particular from temperature increases in the sample being inspected are ruled out. At the same time, the sample is subjected to radiation in a spectral range corresponding to the solar radiation that generally causes the variations in the properties of the materials being inspected.
If necessary, the sample can be subjected to visual or IR light in order to additionally set a defined temperature on or inside the sample without contact.